The drive belt is hidden behind a 6mm MDF guard plate, cut to the largest size possible, to prevent fingers jamming in the pulleys. The pulleys are both double, but I need only one belt.

Here you see also the clamps which currently serve as an adjustment mechanism. A bit sub-optimal, but it's not as bad as it looks. I've worked out several screw-adjustment mechanisms, but none of them are likely to be as rigid as I'd like.